The function of an HUD is to enable the driver or pilot to see important information without losing sight of the outside environment. To provide this function, the information obtained from an imager is projected onto a semi-transparent optical element, also called a combiner, using an optical system or relay optic. In daytime use, such information is essentially symbology. After reflection on the combiner, the optical beams are collimated, that is to say projected to infinity so that the pilot can view the symbology and the landscape at the same time without having to adjust visually.
There are two types of combiners. The combiners of the first type are flat plates with no optical power. The combiners of the second type are curved plates that have a certain optical power. Hereinafter in the description, the term “eye box” will be used to designate the region in which the collimated image that is to be seen by the pilot is formed.
The configuration of certain aeroplane cockpits equipped with an HUD is such that, in certain conditions, a portion of the solar radiation may be focused on the surface of the imager, inside the very HUD. This occurs when the solar rays follow the reverse path of the light obtained from the display, either by passing through the combiner or by being reflected thereon. The first transmission case is a problem only if the combiner is a flat plate. The second reflection case is illustrated in FIG. 1. The display or HUD of FIG. 1 comprises an imager 1, a relay optic 2 comprising a dioptric assembly 21 and a return mirror 22 and a curved combiner 3. When the imager is passive, as is the case with liquid crystal screens, it may include a light box 5 which provides the illumination for the imager. As can be seen in this FIG. 1, if the solar rays 11 penetrate the eye box and are then reflected on the combiner 3, they are focused on the imager 1. They can penetrate the eye box only when the pilot is not there, that is to say, for example, in flight when he moves his head or even on the ground when there is no pilot in the cockpit. In both cases, the solar rays then follow the reverse path of that of the information projected on the combiner, which means that they pass through the optical system and are then focused on the information display surface. This display surface can be of different types, more or less resistant to solar illumination. They can then be damaged, reversibly or not, by the solar radiation entering the HUD. It is therefore essential to provide protection for the HUD against solar illumination.
To avoid this problem, the existing solutions consist in reducing the transmission and/or reflection coefficients of the optical system, permanently or as a function of the received solar illumination.
The first solution described in the patent “Method and apparatus for attenuating solar flux in a Head-Up Display” (U.S. Pat. No. 6,574,048 B2) is based on reducing the reflection coefficient of a mirror forming the optical system of an HUD. The induced loss of brightness is offset by an increase in the luminance of the light source, which has the drawback of increasing electrical consumption.
In a second solution described in the patent “Electrically dimmable combiner optics for head-up display” (WO/2008/109231 A2), the transmission from the combiner is controlled electrically. This method is used initially to increase the contrast of the HUD in the presence of a strongly lit landscape by reducing the brightness of the outside landscape without reducing the brightness of the symbology, since the reflection coefficient of the combiner is unaffected by the variation of the transmission coefficient. This technique is therefore applicable in the case of solar rays passing through a flat combiner, but is not applicable in the case of reflection on the combiner.
A third solution described in the patent “Diffraction Head-Up Display solar radiation filter” (WO/1984/002197) consists in introducing, into or very close to the object focal plane, between the information display surface and the optical system, a transmissive array of liquid crystal photodetectors or of a photochromic material, that is to say which darkens or lightens according to the illumination received. The major drawback with this method is the time taken by the photochromic material to become transparent again when it is no longer receiving solar illumination. The result of this is a region in which the information is no longer displayed for a certain time, and this can be a considerable nuisance for the pilot.